body{color: #fff;text-align: center;}
header,nav,main,footer{
    width: 700px;
    margin: 0 auto;
    background-color: #5f5f5f;
}
header{padding: 10px 0;}
nav ul{
    padding: 0;
    /* 清除浮动：给浮动标签的父标签添加高度 */
    overflow: hidden;
    margin: 10px 0;
}
nav ul li{
    list-style: none;
    float: left;
    width: 19%;
    border-right: 1px #000 solid;
    margin: 10px 0;
}
nav ul .a{color: rgb(11, 163, 209);}
nav ul .b{border: none;}
main{
    overflow: hidden;
    margin: 10px auto;
}
main section{
    float: left;
    width: 49%;
    margin: 10px 0;
}
main .c{border-right: 1px #000 solid;}
footer{padding: 10px 0;}